i

SIMULATION OF ACTIVE POWER FILTER USING MATLAB/SIMULINK

NUR IZZATI NADIAH BINTI ISHAK

Bachelor of Electrical Engineering(Industrial Power)

May 2010

ii

“ I hereby declared that I have read through this report and found that it has comply

the partial fulfillment for awarding the degree of Bachelor of Electrical Engineering

(Industrial Power)”

Signature :

Supervisor’s Name : EN. AZHAR BIN AHMAD

Date :

iii

SIMULATION OF ACTIVE POWER FILTER USING MATLAB/SIMULINK

NUR IZZATI NADIAH BINTI ISHAK

This report is submitted in partial fulfillment of requirement for degree of Bachelor in

Electrical Engineering (Industrial Power)

Faculty of Electrical Engineering

Universiti Teknikal Malaysia Melaka

iv

May 2010

“ I hereby declared that this report is a result of my own work except as cited clearly

in the references ”

Signature :

Name : NUR IZZATI NADIAH BINTI ISHAK

Date : 11 MAY 2010

April 2010

22 APRIL 2010

v

ACKNOWLEDGEMENT

First and foremost, I thank Allah the Almighty for blessing me to complete my

Final Year Project 1. I would like to enlarge my appreciation to En. Azhar bin Ahmad

because of the kindness heart to accept me as one of the student under his supervision.

Special thanks also dedicated to her for all comments, idea and guideline begin from the

first day I start this project.

This appreciation also goes to my friends that always give me support, opinion,

sharing ideas and advices for me to complete this project.

To my beloved family, I would like to forward my obliged to them for their

continuous support during my study period, their patience and benevolence. Lastly, I

would like to thank everyone who has contributed during my Final year Project 1. Your

kindness and cooperation in completion of my paper work is much appreciated.

Thank You

vi

ABSTRACT

This project was about the simulation of active power filter using Matlab software. The

aim of this project is to review an active power filter that commonly used to mitigate

harmonics. Harmonics are AC voltage or currents whose frequency is some integral

multiple of the fundamental frequency. Harmonics degrade the level of power quality and

efficiency in a industrial and commercial facility. Thus, to solve this harmonics problems,

there are new class of harmonic mitigation devices injects a mirror image waveform of the

harmonic portion of the distorted waveform which is active power filter. Active power

filters are relatively new and rather costly but have important advantages that should be

watched carefully which are inherently current limiting, have no resonance problems,

intelligent and adaptable, can be configured to either correct the full spectrum of harmonics

or to target specific harmonics and being able to compensate for harmonics without

fundamental frequency reactive power concerns. The system is verified by simulation

using Matlab/Simulink simulation package. The paper starts with a brief overview of

harmonic distortion problems and their impacts on electric power quality, how the active

power filter solved this problems and the verifying using Matlab software. As the result the

harmonics problems can be solved using active power filter.

vii

ABSTRAK

Projek ini adalah berkaitan dengan simulai penapis kuasa aktif yang menggunakan perisian

Matlab. Projek ini bertujuan untuk mengkaji tentang penapis kuasa aktif yang sering

digunakan untuk mengurangkan kesan harmonik. Harmonik adalah voltan atau arus ulang-

alik di mana frekuensinya terdiri daripada gandaan frekuensi asas.Harmonik merendahkan

tahap kualiti kuasa dan kecekapan dalam kemudahan industri dan komersial. Jadi, untuk

menyelesaikan masalah harmonik, terdapat alat baru untuk mengurangkan kesan harmonic

di mana ia berfungsi dengan menyuntik gelombang imej cermin ke gelombang herot yang

terkena kesan harmonic iaitu penapis kuasa aktif. Penapis kuasa aktif masih baru dan agak

mahal tetapi mempunyai beberapa kelebihan penting yang patut dinilai iaitu secara semula

jadinya ia menghadkan arus, tiada masalah resonans(gema), bijak dan boleh diubah-suai,

boleh ditatarajah samada pada semua spectrum atau pada kawasan sasaran tertentu

harmonik dan juga ia boleh mengganti untuk harmonic tanpa frekuensi kuasa reaktif yang

asas.Sistem ini disahkan dengan simulasi menggunakan perisian simulasi Matlab/Simulink.

Laporan ini bermula dengan ulasan kajian tentang masalah gangguan harmonik dan

kesannya pada kualiti kuasa elektrik, bagaimana penapis kuasa aktif menyelesaikan

masalah ini dan pengesahan projek ini menggunakan perisian simulasi Matlab. Hasilnya,

masalah harmonik dapat diselesaikan dengan menggunakan penapis kuasa aktif.

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TABLE OF CONTENTS

CHAPTER TITLE

PAGE

Supervisor Declaration i Title page ii Declaration iii Acknowledgement iv Abstract v Abstrak vi Table of Content vii-viii List of Figures

ix-x

1 INTRODUCTION

1.1 Introduction of the project 1 1.2 Problem Statement 2 1.3 Project Objective 2 1.4 Scope of project

3

2 LITERATURE REVIEW

2.1 Introduction 4 2.2 Power Quality 4-7 2.3 Harmonics 7-11 2.4 Active Power Filters 12-14 2.5 Shunt Active Power Filter 14 2.6 Operation of an Active Power Filter

2.7 Power switching devices 15-16

17 2.8 Matlab/Simulink Software 18

3 METHODOLOGY

3.1 Introduction 19 3.2 Project Flow Chart 20 3.3 Literature Review 21 3.4 Study Software(Matlab/Simulink) 21

ix

3.5 Design Circuit and Controller 21 3.6 Simulate 21 3.7 Report writing 22

4 RESULT

4.1 Circuit Design 23 4.2 Result

4.2.1 Main Circuit 4.2.2 Non Linear Load 4.2.3 Active Power Filter Circuit

24 24-25 26-27 28-30

5 ANALYSIS AND DISCUSSION

5.1 Active Power Filter Theory 31-32 5.2 Active Power Filter Circuit Design

5.3 Main Circuit 5.4 Main Circuit and Load 5.5 Active Power Filter Circuit 5.6 Hysteresis Controller Circuit 5.6.1 How to generate reference signal? 5.6.2 Upper Comp and Lower Comp 5.6.3 Pulse Triggering Wave 5.6.4 Active Power Filter Current

32-33 34 35 36 37

37-41 42-43 43-45 46-48

5 CONCLUSION

5.1 Introduction 49 5.2 Conclusion 49 5.3 Recommendation

50

REFERENCES 51-52

x

LIST OF FIGURES

FIGURE TITLE

PAGE

2.1 Components of a typical active power filter system 13 2.2 Condition of an active power

filter:series,parallel,hybrid 14

2.3 Single phase shunt active power filter 15 2.4 Cancellation of harmonic from non linear load 16 2.5 2.6

Power switching devices comparison Igbt pulse width modulation inverter

16 17

2.7 Matlab logo 18 3.1 Project flow chart 20 4.1 Design circuit 23 4.2 Main circuit 24 4.3 Source voltage 24 4.4 Source current 25 4.5 THD main circuit 25 4.6 Main circuit connected to load 26 4.7 4.8 4.9 4.10 4.11 4.12 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 5.10 5.11 5.12 5.13 5.14 5.15 5.16 5.17

Load current Load voltage Main circuit with Active Power Filter Hysteresis input Pulse wave Active Power Filter current APF Block Diagram Active Power Filter design circuit Main circuit Source current Main circuit connected to rectifier load Load current Active Power Filter circuit Hysteresis Controller circuit Reference signal IFFT output Vref Multiply signal between IFFT and Vref Input of hysteresis controller Load current Multiply signal Input signal hysteresis controller Upper Comparator and Lower Comparator

26 27 28 29 29 30 31 33 34 34 35 35 36 37 37 38 38 39 39 40 40 41 42

xi

5.18 5.19 5.20 5.21 5.22 5.23 5.24 5.25 5.26 5.27 5.28

Lower Comparator Input Upper Comparator Input SR Flip Flop Input SR Flip Flop Output SR Flip Flop IGBT Circuit Design APF Current Measurement Active Power Filter Current Active Power Filter Current(theory) Load current Active Power Filter Current

42 43 43 44 44 45 46 47 47 48 48

1

CHAPTER 1

INTRODUCTION

1.1 Introduction of the project

The past several years have seen a rapid increase of power electronics-based loads

connected to the distribution system. These types of loads draw nonsinusoidal current from

the mains, degrading the power quality by causing harmonic distortion. These nonlinear

loads appear to be prime sources of harmonic distortion in a power distribution system. In

addition, the harmonic currents produced by nonlinear loads can interact adversely with a

wide range of power system equipment, most notably capacitors, transformers, and motors,

causing additional losses, overheating, and overloading and interferences.

Harmonic distortion in power distribution systems can be suppressed using two

approaches namely, passive and active powering. Passive filters are known to cause

resonance, thus affecting the stability of the power distribution systems. The basic

principle of active power filter is to utilize power electronics technologies to produce

specific currents components that cancel the harmonic currents components caused by the

nonlinear load. Active power filter have a number of advantages over the passive filters.

First of all, they can suppress not only the supply current harmonics, but also the reactive

currents. Moreover, unlike passive filters, they do not cause harmful resonances with the

power distribution systems. The active filter uses power electronic switching to generate

harmonic currents that cancel the harmonic currents from a nonlinear load.

2

The result is verify using Matlab/Simulink simulation package which the expected

result achieved when the harmonics problem that produced by non linear load solved using

an active power filter.

1.2 Problem statement

• Harmonics has become the common problem nowadays.

Nowadays there are many modern electronic equipment such as personal or

notebook computers, laser printers, fax machines, telephone systems, stereos,

radios, TVs, adjustable speed drives, and any other equipment powered by

switched-mode power supply (SMPS) equipment. This switch mode power supply

equipment is also referred to as non-linear loads. This type of non-linear loads or

SMPS equipment generates the harmonics they’re sensitive to and that originate

right within your building or facility. SMPS equipment generates electrical non-

linear load in most electrical distribution systems. There are two types of non-linear

loads: single-phase and three-phase.

• Harmonics is not mitigating properly and need an effective solution.

Not all harmonic mitigation solution is ideal for every application. There has many

ways to mitigate harmonics depend on the cost and load.Active power filter is one

of the best solution to mitigate harmonics and suitable for all application.

• Simulation of an active power filter is still an open problem.

Although there were many research and simulation based on active power filter,

there still have no absolute method in order to mitigate harmonics effectively.

1.3 Project Objectives

• Able to study about active power filter.

• Able to design circuit of the active power filter.

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• Able to mitigate harmonics effectively.

• Simulation of active power filters to verify its effect in mitigating harmonics.

1.4 Scope of the project

• Simulate single-phase active power filter to eliminate harmonic distortion.

• The circuit of single-phase active power filter will be simulating using Matlab

Simulation package.

4

CHAPTER 2

LITERATURE REVIEW

2.1 Introduction

This chapter will discuss about the literature review of the project. The

sources of the information had been gathered from books, journal, research papers,

encyclopedias, newspapers, magazines, handbooks, thesis, bibliographies and World Wide

Web (WWW).Fact and finding is the formal process to collect and capture the entire

information about system, system requirements and system preferences. Fact and finding is

most crucial to the system planning and system analysis phase. It helps to learn about the

vocabulary, problems, opportunities, constraints, requirements and priorities of a business

and a system.

The literature review starts with the theory of power quality problems, continue

with the harmonics which one of the main factor that degrading the power quality level.

Then, there are explanation about several ways to mitigate harmonics then continue with

the theory about an active power filter that use to mitigate harmonics and the circuit

connection of an active power filter that been chosen for the project. Finally, there are brief

explanation about the software that been used in this project to verify the result which is

Matlab software.

2.2 Power Quality

Power quality is simply the interaction of electrical power with electrical

equipment. If electrical equipment operates correctly and reliably without being

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damaged or stressed, we would say that the electrical power is of good quality.( Heydt,

G.T. (1991) .On the other hand, if the electrical equipment malfunctions, is unreliable,

or is damaged during normal usage, we would suspect that the power quality is poor.

As a general statement, any deviation from normal of a voltage source (either DC or

AC) can be classified as a power quality issue. Power quality issues can be very high-

speed events such as voltage impulses / transients, high frequency noise, wave shape

faults, voltage swells and sags and total power loss. The power quality (PQ) problems

in power utility distribution systems are not new, but only recently their effects have

gained public awareness. Power quality means to maintain purely sinusoidal current

waveform in phase with purely sinusoidal voltage waveform(Bollen, Math H.J.

(2000)).In its broadest sense, power quality is a set of boundaries that allows electrical

system to function in their intended manner without significant loss of performance or

life (Dugan, Roger C)

Advances in semiconductor device technology have fuelled a revolution in power

electronics over the past decade, and there are indications that this trend will continue

.However the power electronics based equipments which include adjustable-speed

motor drives, electronic power supplies, DC motor drives, battery chargers, electronic

ballasts are responsible for the rise in PQ related problems. These nonlinear loads

appear to be prime sources of harmonic distortion in a power distribution system.

Harmonic currents produced by nonlinear loads are injected back into power

distribution systems through the point of common coupling (PCC). As the harmonic

currents pass through the line impedance of the system, harmonic voltages appear,

causing distortion at the PCC. Ideally, voltage is supplied by a utility as sinusoidal

having an amplitude and frequency given by national standards or system

specifications with an impedance of zero ohms at all frequencies. No real-life power

source is ideal and generally can deviate in at least the following ways; this is several

of power quality problems which are:

• Variations in the peak or RMS voltage are both important to different types of

equipment. When the RMS voltage exceeds the nominal voltage by 10 to 80% for

0.5 cycles to 1 minute, the event is called a "swell".

6

• A "dip" (in British English) or”sag" (in American English - the two terms are

equivalent) is the opposite situation: the RMS voltage is below the nominal voltage

by 10 to 90% for 0.5 cycles to 1 minute.

• Random or repetitive variations in the RMS voltage between 90 and 110% of

nominal can produce a phenomenon known as "flicker" in lighting equipment.

Flicker (light flicker) is the impression of unsteadiness of visual sensation induced

by a light stimulus on the human eye. A precise definition of the voltage

fluctuations (voltage flicker) that produce light flicker that annoys humans, has

been subject to ongoing debate in more than one scientific community for many

years.

• Abrupt, very brief increases in voltage, called "spikes", "impulses", or "surges",

generally caused by large inductive loads being turned off, or more severely by

lightning.

• "Undervoltage" occurs when the nominal voltage drops below 90% for more than 1

minute. The term "brownout" is an apt description for voltage drops somewhere

between full power (bright lights) and a blackout (no power - no light). It comes

from the noticeable to significant dimming of regular incandescent lights, during

system faults or overloading etc., when insufficient power is available to achieve

full brightness in (usually) domestic lighting. This term is in common usage has no

formal definition but is commonly used to describe a reduction in system voltage

by the utility or system operator to decrease demand or to increase system

operating margins.

• "Overvoltage" occurs when the nominal voltage rises above 110% for more than 1

minute.

• Variations in the frequency

• Variations in the wave shape - usually described as harmonics

• Nonzero low-frequency impedance (when a load draws more power, the voltage

drops)

• Nonzero high-frequency impedance (when a load demands a large amount of

current, then stops demanding it suddenly, there will be a dip or spike in the voltage

due to the inductances in the power supply line) .

7

Each of these power quality problems has a different cause. Some problems are a

result of the shared infrastructure. For example, a fault on the network may cause a dip that

will affect some customers and the higher the level of the fault, the greater the number

affected, or a problem on one customer’s site may cause a transient that affects all other

customers on the same subsystem.( Dugan, Roger C et al (2003).

In this project, we will discuss mainly about the power quality that causes by the

harmonics. For harmonic problems, arise within the customer’s own installation and may

or may not propagate onto the network and so affect other customers. Harmonic problems

can be dealt with by a combination of good design practice and well proven reduction

equipment

2.3 Harmonics

Harmonics are currents or voltages with frequencies that are integer multiples of

the fundamental power frequency being 50 or 60Hz (50Hz for European power and 60Hz

for American power). For example, if the fundamental power frequency is 60 Hz, then the

2nd harmonic is 120 Hz, the 3rd is 180 Hz, etc. In modern test equipment today harmonics

can be measured up to the 63rd harmonic. When harmonic frequencies are prevalent,

electrical power panels and transformers become mechanically resonant to the magnetic

fields generated by higher frequency harmonics. When this happens, the power panel or

transformer vibrates and emits a buzzing sound for the different harmonic frequencies.

Harmonic frequencies from the 3rd to the 25th are the most common range of frequencies

measured in electrical distribution systems. Harmonics are electric voltages and currents

that appear on the electric power system as a result of certain kinds of electric loads.

Harmonic frequencies in the power grid are a frequent cause of power quality problems.

(Sankaran, C. (1995-10-01). "Effects of harmonics on power systems”).A harmonic is the

term used for unwanted and possibly destructive current flow on your facilities conductors

(wiring). They are most prevalent on your neutral conductors. (Dan Maxcy

(2001).”Harmonics”.)

Additionally, harmonics are caused by and are the by-product of modern electronic

equipment such as personal or notebook computers, laser printers, fax machines,

8

telephone systems, stereos, radios, TVs, adjustable speed drives and variable frequency

drives, battery chargers, UPS, and any other equipment powered by switched-mode power

supply (SMPS) equipment. The above-mentioned electronic SMPS equipment is also

referred to as non-linear loads. This type of non-linear loads or SMPS equipment

generates the very harmonics they’re sensitive to and that originate right within your

building or facility. SMPS equipment typically forms a large portion of the electrical non-

linear load in most electrical distribution systems.

There are basically two types of non-linear loads: single-phase and three-phase.

Single-phase non-linear loads are prevalent in modern office buildings while three-phase

non-linear loads are widespread in factories and industrial plants.

In an electrical distribution system harmonics create:

1. Large load currents in the neutral wires of a 3 phase system. Theoretically the

neutral current can be up to the sum of all 3 phases therefore causing overheating of

the neutral wires. Since only the phase wires are protected by circuit breakers of

fuses, this can result in a potential fire hazard.

2. Overheating of standard electrical supply transformers which shortens the life of a

transformer and will eventually destroy it. When a transformer fails, the cost of lost

productivity during the emergency repair far exceeds the replacement cost of the

transformer itself.

3. High voltage distortion exceeding IEEE Standard 1100-1992 "Recommended

Practice for Powering and Grounding Sensitive Electronic Equipment" and

manufacturer’s equipment specifications.

4. High current distortion and excessive current draw on branch circuits exceeding

IEEE Standard 1100-1992 "Recommended Practice for Powering and Grounding

Sensitive Electronic Equipment" and manufacturer’s equipment specifications.

5. High neutral-to-ground voltage often greater than 2 volts exceeding IEEE Standard

1100-1992 "Recommended Practice for Powering and Grounding Sensitive

Electronic Equipment."

6. High voltage and curent distortions exceeding IEEE Std. 519-1992 "Recommended

Practices and Requirements for Harmonic Control in Electrical Power Systems."

7. Poor power factor conditions that result in monthly utility penalty fees for major

users (factories, manufacturing, and industrial) with a power factor less than 0.9.

9

8. Resonance that produces over-current surges. In comparison, this is equivalent to

continuous audio feedback through a PA system. This results in destroyed

capacitors and their fuses and damaged surge suppressors which will cause an

electrical system shutdown.

9. False tripping of branch circuit breakers.

10. Malfunctioning of microprocessor-based equipment.

11. Overheating in neutral conductors. transformers, or induction motors.

12. Deterioration or failure of power factor correction capacitors.

13. Erratic operation of breakers and relays.

14. Pronounced magnetic fields near transformers and switchgear.

There are several solutions to compensate for and reduce harmonics, which are:

1. Oversize the neutral wiring.

In modern facilities, the neutral wiring should always be specified to be the same

capacity as the power wiring, or larger—even though electrical codes may permit under-

sizing the neutral wire. An appropriate design to support a load of many personal

computers, such as a call center, would specify the neutral wiring to exceed the phase wire

capacity by about 200 percent. Particular attention should be paid to wiring in office

cubicles. Note that this approach protects the building wiring, but it does not help protect

the transformers.

2. Use separate neutral conductors.

On three-phase branch circuits, instead of installing a multi-wire branch circuit

sharing a neutral conductor, run separate neutral conductors for each phase conductor. This

increases the capacity and ability of the branch circuits to handle harmonic loads. This

approach successfully eliminates the addition of the harmonic currents on the branch

circuit neutrals, but the panel board neutral bus and feeder neutral conductor must still be

considered.

3. Use DC power supplies, which are not affected by harmonics.

In the typical data center, the power distribution system converts 480-volt AC

utility power through a transformer that steps it down to 208-volt AC power that feeds

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racks of servers. One or more power supplies within each server convert this AC input into

DC voltage appropriate for the unit’s internal components. These internal power supplies

are not energy efficient, and they generate substantial heat, which puts a costly burden on

the room’s air conditioning system. Heat dissipation also limits the number of servers that

can be housed in a data center. According to a recent article in Energy and Power

Management magazine, ''Computers and servers equipped with DC power supplies instead

of AC power supplies produce 20 to 40percent less heat, reduce power consumption by up

to 30 percent, increase server reliability ,offer flexibility to installations, and experience

decreased maintenance requirements.”That sounds good, but when cost, compatibility,

reliability and efficiency are considered together, the move from AC to DC power is not

justified for most data centers. AC power—even though it is slightly less efficient is

universally acceptable to existing equipment.

4. Use K-rated transformers in power distribution components.

A standard transformer is not designed for high harmonic currents produced by

non-linear loads. It will overheat and fail prematurely when connected to these loads.

When harmonics were introduced into electrical systems at levels that showed detrimental

effects (circa 1980), the industry responded by developing the K-rated transformer. K-

rated transformers are not used to handle harmonics, but they can handle the heat generated

by harmonic currents and are very efficient when used under their K-factor value. K-factor

ratings range between 1 and 50. A standard transformer designed for linear loads is said to

have a K-factor of 1. The higher the K-factor, the more heat from harmonic currents the

transformer is able to handle. Making the right selection of K-factor is very important,

because it affects cost and safety. The table shows appropriate K-factor ratings to use for

different percentages of non-linear current in the electrical system.

5. Passive filters

Passive filters (also called traps) include devices that provide low-impedance paths

to divert harmonics to ground and devices that create a higher-impedance path to

discourage the flow of harmonics. Both of these devices, by necessity, change the

impedance characteristics of the circuits into which they are inserted. Another weakness of

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passive harmonic technologies is that, as their name implies, they cannot adapt to changes

in the electrical systems in which they operate. This means that changes to the electrical

system (for example, the addition or removal of power factor–correction capacitors or the

addition of more nonlinear loads) could cause them to be overloaded or to create

“resonances” that could actually amplify, rather than diminish, harmonics.

Although simple and least expensive, the passive filter inherits several

shortcomings. The filter components are very bulky because the harmonics that need to be

suppressed are usually of the low order. Furthermore the compensation characteristics of

these filters are influenced by the source impedance. As such, the filter design is heavily

dependent on the power system in which it is connected to. Passive filters are known to

cause resonance, thus affecting the stability of the power distribution systems. Frequency

variation of the power distribution system and tolerances in components values affect the

filtering characteristics. The size of the components become impractical if the frequency

variation is large. As the regulatory requirements become more stringent, the passive filters

might not be able to meet future revisions of a particular Standard. This may required a

retrofit of new filters.

6. Active power filters

This is the most versatile and flexible solution compared to others. Like an

automatic transmission in a car, active filters are designed to accommodate a full range of

expected operating conditions upon installation, without requiring further adjustments by

the operator. Active harmonic filters are relatively new and rather costly, but offer several

advantages. They are inherently current limiting, have no resonance problems, are

"intelligent" and adaptable, and can be configured to either correct the full spectrum of

harmonics or to target specific harmonics. Although this technology is new, it has

important advantages and should be watched carefully. It is worth stressing that the

particular solution for your facility must be the result of careful analysis and isolation of

the problem.

From above options of solution to mitigate harmonics, using active power filter is

chosen based on the advantages and the effect to the electrical system.(Anynomous,1992).

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2.3 Active power filters

Active power filter applicable to compensate current-based distortions such as

current harmonics, reactive power and neutral current. It is also used for voltage-based

distortions such as voltage harmonics, voltage flickers, and voltage sag and swell and

voltage imbalances. Active filters have the advantage of being able to compensate for

harmonics without fundamental frequency reactive power concerns. This means that the

rating of the active power can be less than a comparable passive filter for the same non-

linear load and the active filter will not introduce system resonances that can move a

harmonic problem from one frequency to another.

Remarkable progress in power electronics had spurred interest in APF for harmonic

distortion mitigation. The basic principle of APF is to utilize power electronics

technologies to produce specific currents components that cancel the harmonic currents

components caused by the nonlinear load.

Figure 2.1: Components of a typical APF system

Figure 2.2 shows the components of a typical APF system and their connections.

The information regarding the harmonic currents and other system variables are passed to

the compensation current/voltage reference signal estimator. The compensation reference

signal from the estimator drives the overall system controller. This in turn provides the

13

control for the gating signal generator. The output of the gating signal generator controls

the power circuit via a suitable interface. Finally, the power circuit in the generalized block

diagram can be connected in parallel, series or parallel/series configurations depending on

the interfacing inductor/transformer use.

APFs have a number of advantages over the passive filters. First of all, they can

suppress not only the supply current harmonics, but also the reactive currents .Moreover,

unlike passive filters; they do not cause harmful resonances with the power distribution

systems. Consequently, the APFs performances are independent on the power distribution

system properties. On the other hand, APFs have some drawbacks. Active filtering is a

relatively new technology, practically less than four decades old. There is still a need for

further research and development to make this technology well established.

Active power filter can be connected in several power circuit configurations as

illustrated in the block diagram shown in Figure 2.2. In general, they are divided into three

main categories, namely shunt APF, series APF and hybrid APF.

Series connection of active power filter Parallel connection of an

active power filter

Hybrid connection of an active power filter

Figure 2.2: Connection of an active power filter; series, parallel and hybrid